Ion source



March 16, 1954 Q BERRY 2,672,560

ION SOURCE Filed Oct. 27, 1952 2 Sheets-Sheet l Fla. 2. 34x. (29 mac/1s INLET .36 I

T0 EXHAUST J- INVENTOR. a9 s40 2o cL/FFolw c. BERRY jmwi A T TORNE V C. E. BERRY ION SOURCE March 16, 1954 2 Sheets-Sheet 2 Filed Oct. 27, 1952 ne xbq EOhwbnFiQ INVENTOR. CLIFFORD E. BERRY ATTORNEY Patented Mar. 16, 1 954 U N I TE D "E S f 2,672,560

IUN SOURCE Clilford; E. -Berry, Altadena, Galif., assignor to Consolidated Engineering QgjrporatiOn, Pasa- 'aena, Califi, a corporationof California Application October 27, 1952;"S'e rial NO-'*317)1'06 9 -Claims. (crest-41 n) $1 This invention is directed 1 to an ion source characterized by development of a high intensity ion beam. The-ion source finds use in mass spectrometry, ionization gauges, ionic vacuum pumps and the like.

In the presently conventional form of ion source, ions are developed 'by electron bombardment of an ionizable medium. Usually an electron beam is directed across an ionizing" region from an electron source 'or gun adjacent one side of the region to an electron target adjacent an opposite side. Theelectrons traversingthe-re- -gion'are discharged at the target. "Theelect'ron beam in such instance traverses the ionizing region once only. V

As the pres'sureof an ionizable' gas in the ionization region is reduced, the probability of an electron striking a'gas particle decreases linearly. As a consequence at verylow pressures'towhich ion sources are general-lyexpo'sed; very few electrons make an ionizing' coll'ision. 'By -way of example, in nit'rogenata pressure of 10- mm. Hg 'themean free path of electronisabout 300 meters, while the path of an-electron in a typical ion source is 'approximately 3 cm. Therefore, on the average,- only'one electron in 10,000 will make an ionizing collision.

To increase the efliciency 'of ionization it is necessary to increase the probability of ionizing collisions. This end may be achieved at any given pressure only by increasing the-intensity of the electron beam or by in'c'reasing thedistance that each electron travels in theionizing region. But there arev practical-limitations on the extent to which the intensity of-an; electron beam can be increased as'determined by power consumption, filament size; overheating, and the like. By increasing the lengthof the" path-of an electron inan ionizing region the probability of any given electron "striking a gas molecule is increased accordingly, and the same net 'result is accomplished as would be possible from a corresponding multiplication of electron 'intensity. v

I have developed an ion source such-that each electron may traverse theregion ofionization many times. An ion soiir'cein accordance with the invention comprises means defining an annular ionization chamber,"a magnet shaped to develop a radial magnetic field across thechamber,an electron emitting filament'disp'osed concentrically with respect to the chamber-adjacent either the outer or inner oircumferen-ce'of the annulus, means for directing the "electrons emitted from the -filament 'radiallyacross' the chamber, means disposed in the plane of- -the filament and on-the= opposite-=side of tli'e anniilar chamber for redirecting 'suchielectrons -radially aback toward.- the filament; and means expel-ling ions fromthe'ionizationechamber. v.

" shell "I 4 'and in the plane of =the cof-planai 'slots The filament -22 Y discontinuous i and adjoining ends' thereofare connected by--leac ls is mounted in theanhulus V "shell M and is "supported th ein by' insulating 45...

In this type of ion source electrons travel back and forth between the filament and 'the means for redirecting the: electrons, the 1 energy -of I the reference to the following-detailed description taken in conjunction with the accompanying drawing, in which:

Fig. 1 is a front elevation of-an ion 'source in accordance with the-invention;

Fig. 2 is'a vertical-section taken on the line 22 of Fig. 1; and

Fig. *3 is a partial vertioal secti'on throng-man evacuating system "including anionic 'vacuum pump embodying the invention.

The ionsource illustrated in tFigss 1 lama-comprises a cylindrical magnet F1 o -having a' 'ce'n-tral coaxial core I I and defining'an annulus -I2 across which a radial magnetic-'field'is developedby the *magnet. An annular metal shelL-"M is-suppo'rted within the "annulus I 2 by iiisulatingisposts #15 connected between themagnet- I U and 'shIL M.

The shell *is provided wi'th co-planar inner and outer circumferential.slots 18, 19,-*'-respectively,

which are continuoussave for a miiiimum number of structural webs 2-8.

An electron emitting filament 121.1 01 generally circular configuration is mountedconcentrically around magnet core ll interiorly of thewannulus 2'3, 24 across a power source' 25. Gonventionally, the leads 23,-24'are' constructed in the or'i'n' 'of support posts to =support= the"- filament within the magnet. A cylindrical-electron reflector 283 2'1 'ex'teriorly of the posts 29 extending from the magnet, "the -arrangement "being such that ineqeiectron fi1am'eno'22, i'the inner and outer peripheral slots "I 8 and 1 9 Of the shell I11 andthe cyliiitifi'cal'lfl'fit't) -28 I are-in a common radial-plane from filament '2 2- may pass suecessiv'ely' thr'ough Electrons emitted =tion by an" insulating post 3|. 'I'heg-rid 3ll may bea helical coil/ as -"sho\vn;:"separate concentriically arranged circularconductors; or an -z annuelar" screen, the: function: or. theigrid being? de- Ws'cri-bd hereinafter.

The mneemeecsadepredwee emumeammn an envelope 34 which may, for example, form a part of a mass spectrometer system and includes an exhaust line 35 adapted to be connected to an evacuating system (not shown) and a gas inlet line 36 for admitting into the system a gas to be ionized. I

Filament 22 is biased negatively with respect to the shell M by means of a potential source 38 and at the same time is biased positively with respect to the reflector 28 by means of a potential source 39. The grid 30 is biased negatively with respect to the shell M by means of a potential source 40, the effect of the relative potentials of the several elements being made apparent from the following description of the operation of the ion source.

Electrons are emitted from filament 22 and, under the influence of the potential existing between the filament and the shell 14, are propelled through the shell slit l8 and travel radially across the annulus defined by the shell. The radial magnetic field has the effect of constraining the electrons very nearly in the plane of the filament so that in traveling across the ionization chamber they are directed toward the outer peripheral slit l9 thereof and approach the reflector 28. However, because of the combined action of the magnetic field and the electrical field between the shell and the grid 30, the electrons undergo an angular deflection in the plane of the filament. After passing through the outer peripheral slit 19 they are reflected back into the chamber as a consequence of the repelling potential developed between the reflector 28 and the shell. Again such a reflected electron will undergo a further angular deflection in the same direction and in the same plane. If the filament is operated on direct current and the polarity is correctly established, a given electron returning to the filament after traveling across the ionization chamber and back inherently approaches a point on the filament more negative than that from which it was emitted so that it is again reflected. As a consequence, any given electron will tend to make repeated traversals of the annulus defined by the shell l4 while at the same time migrating around the axis of the shell in a direction toward the negative terminal of the electron filament 22.

As ions are formed within the annulus defined by shell 14 they are expelled therefrom under the influence of the propelling potential between shell [4 and grid 30. The resulting beam is of annular section as defined by the radial section of shell l4 and is propelled from the shell in a direction paralleling the longitudinal axis thereof.

A characteristic electron path is plotted in part in Fig. l as the dotted line 42. Each time an electron traverses the annular ionizing region in excess of its first crossing, the effect is the same as a like multiplication of the intensity of electron emission at the filament. Thus, in a conventional ion source, many times the normal power requirement would be required to develop an ion beam of an intensity corresponding to that produced in the present ion source by multiple-stage electron traversal of the ionizing region. Actually, a given electron, unless undergoing an ionizing collision, will traverse the ionizing region many hundreds of times.

The same operation is effectuated if the electron emitting filament and the reflector interchange in relative position. It is, however, preferable to mount the filament inwardly of the an nular ionizing region, as shown, as minimizing power requirements and heating efiects.

The ion source shown in Figs. 1 and 2 is adapted for use in a mass spectrometer and particularly a mass spectrometer oia type which does not operate on a sharply defined ion beam. The comparativly recently developed high frequency mass spectrometers are representative of the type of instrument in which the present ion source is of great value. The ion source is similarly ideally suited for use in an ionization gauge.

The use of an ion source in accordance with the invention as an ionic vacuum pump in an evacuating system, is illustrated in partial sectional elevation in Fig. 3. The source shown in this figure is identical to that above described and component parts thereof are identified by like reference characters. The source is disposed at an end of a comparatively long tube 50 adapted to connection to a system to be evacuated identified by the reference character 52, the source being interposed between the tube and system with space for gas flow or difiusion from one to the other. The opposite end of the tube 50 is connected serially to a conventional diffusion pump 54 and a fore-pump 56.

The principle of operation of an ionic vacuum pump is well known in the art and the advantages attendant upon the utilization of a high intensity ion beam therein are consequently readily apparent. The various elements of the ion source are maintained at the relative potentials as described in relation to Fig. 2 and in addition the elongated tube 58 of the pump is maintained at a negative potential with respect to grid 30 by means of a potential source 58.

In operation, gas molecules migrating into the annular ionizing region of the source as defined by the shell I4 are ionized and propelled under the influence of the propelling potential between the shell and the grid 30 along the tube 50. The ions are discharged at an end wall 50A of the tube and the resultant neutral particles diffuse both back along the tube 50 and in the direction of diffusion pump 54. Since the tube 50 is purposely made long in comparison with the diffusion pump connection, the majority of the thus neutralized particles find their way out of the system through the diffusion pump.

I claim:

1. An ion source comprising means defining an annular ionization chamber, means developing a radially directed magnetic field across the chamber, an electron emitting filament disposed concentrically with respect to the chamber, means directing electrons emitted from the filament radially across the chamber, means redirecting such electrons radially back toward the filament, and means expelling ions from the ionization chamber.

2. An ion source comprising a metal shell defining an annular ionization chamber, the shell having substantially continuous and radial aligned slots in its inner and outer cylindrical walls, magnet means developing a radially directed magnetic held across the chamber, an electron emitting filament disposed concentrically with respect to the shell and exteriorly of the annulus defined thereby, means directing electrons emitted from the filament radially across the chamber through the radially aligned slots, means redirecting such electrons radially back toward the filament, and means expelling ions from the ionization chamber.

3. Apparatus according to claim 2 wherein said magnet means comprises a cylindrical magnet member closed at one end and having an axial core defining an annulus with the cylindrical magnet wall in which said shell is mounted.

4. An ion source comprising a metal shell defining an annular ionization chamber, magnet means developing a radially directed magnetic field across the chamber, an electron emitting filament disposed concentrically with respect to the shell, means directing electrons emitted from the filament radially across the chamber, a cylindrical reflector disposed concentrically with respect to the shell and opposite the filament for reflecting electrons radially back toward the filament, and means expelling ions from the ionization chamber.

5. Apparatus according to claim 4 wherein said filament comprises a broken ring, the two adjacent ends thereof being adapted to connection across a voltage source.

6. An ion source comprising a metal shell open at one face and defining an annular ionization chamber, magnet means developing a radially directed magnetic field across the chamber, an electron emitting filament disposed concentrically with respect to the shell, means directing electrons emitted from the filament radially across th chamber, a cylindrical reflector disposed concentrically with respect to the shell opposite the filament for reflecting electrons radially back toward the filament, a conductive grid disposed adjacent the open face of the shell, and means maintaining the grid at a negative potential with respect to the shell.

'7. Apparatus according to claim 6 including means maintaining the shell at a positive potential with respect to the filament and the re- 6 flector at a negative potential with respect to the filament.

8. An ion source comprising a generally cylindrical magnet structure closed at one end and having an axial core extending inwardly from the closed end to define an annulus between the core and cylindrical Wall across which a radial magnetic field is maintained, a metal shell mounted in said annulus and comprising inner and outer cylindrical wall portions joined concentrically by an annular closure member extending between radially aligned ends of said wall portions, each wall portion having a substantially continuous circumferential slot therein with the two slots being in radial alignment, an electron emitting filament mounted concentrically with and exteriorly of the shell in radial alignment with an adjacent one of said slots, a cylindrical conductive reflector mounted concentrically with and exteriorly of the shell in radial alignment with and adjacent the other of said slots, a conductive grid disposed adjacent the end of the shell opposite said closure member, a voltage source connected to the filament, and means biasing the filament, shell, reflector and grid so that the shell is at a positive potential with respect to the filament.

9. Apparatus according to claim 8 wherein the filament is mounted concentrically around the magnet core between the core and inner wall member of the shell, and the reflector is mounted concentrically around the shell between the shell and cylindrical magnet wall.

CLIFFORD E. BERRY.

No references cited. 

